refactor: Remove custom JavaProject cache

[adangel]

I've seen some test failure with the error message:

net.sourceforge.pmd.eclipse.ui.dialogs.ViolationDetailsDialogTest.openDialogViaProblemView -- Time elapsed: 0.094 s <<< ERROR!
java.lang.IllegalArgumentException: Path must include project and resource name: /ViolationDetailsDialogTest
	at org.eclipse.core.runtime.Assert.isLegal(Assert.java:68)
	at org.eclipse.core.internal.resources.Workspace.newResource(Workspace.java:2274)
	at org.eclipse.core.internal.resources.Container.getFolder(Container.java:216)
	at net.sourceforge.pmd.eclipse.runtime.properties.impl.ProjectPropertiesImpl.determineBuildPathIncludesExcludes(ProjectPropertiesImpl.java:93)
	at net.sourceforge.pmd.eclipse.runtime.properties.impl.ProjectPropertiesImpl.<init>(ProjectPropertiesImpl.java:83)
	at net.sourceforge.pmd.eclipse.runtime.properties.impl.PropertiesFactoryImpl.newProjectProperties(PropertiesFactoryImpl.java:32)
	at net.sourceforge.pmd.eclipse.runtime.properties.impl.ProjectPropertiesManagerImpl.loadProjectProperties(ProjectPropertiesManagerImpl.java:87)
	at net.sourceforge.pmd.eclipse.ui.dialogs.ViolationDetailsDialogTest.setUp(ViolationDetailsDialogTest.java:59)

I don't think, caching these instances is the correct way. If JavaCore.create() is slow, then the caching should happen in eclipse framework, and not in the PMD plugin itself.

The difference is: The cache might contain a project, whose dependent projects are closed, which could result in wrong analysis. JavaCore.create() opens all dependent projects as a side-effect.

The test failure happens only sporadically. So - if with this change all other tests work find, I'll just merge this PR.